The invention is in the field of examining objects with penetrating radiation such as x-rays, and pertains to scanning an image plane with a beam to form a shadowgraphic image. An important aspect of the invention is to control the beam cross-section during the scan such that the beam footprint does not vary significantly in area with position in the image plane.
In one type of an x-ray machine a scanning x-ray beam passes through an object and scans an image plane in raster fashion. The result is a shadowgraphic x-ray image of the object's interior. Several variants of such an x-ray machine are described in the copending application of the inventor herein, filed on Apr. 14, 1986 and assigned Ser. No. 851,252, which is hereby incorporated by reference in this specification as though fully set forth herein. Further background material is found in the prior art cited in said copending application.
When scanning a beam by means of an apertured collimator, as in the prior art, the area of the beam footprint on the image plane changes during the scan. The term "footprint" is used here to refer to the area of the image plane irradiated at any one time by the scanning beam, disregarding scattered radiation. The term "scanning beam" is used here to refer to a beam of penetrating radiation which has been shaped, as by a collimator, such that it has a selected cross-sectional shape, for example a few mm square or a rectangle of a few mm on one side and a few mm to a few cm on the other.
Referring to FIGS. 1a and 1b for an illustration (not to scale), in FIG. 1a the x-rays emanating from x-ray focal spot 10 are collimated by collimator 12 into a scanning beam 14 which impinges on image plane 16. Such a collimator is sometimes called a fore collimator, meaning that it is between the source and the object plane, to distinguish it from the aft collimator which is sometimes used between the object position and the image plane. As collimator 12 moves up in the plane of the FIG. its angle with respect to central ray of beam 14 changes, for example to the angle illustrated in FIG. 1b. The footprint of beam 14 on image plane 16 is less in area in FIG. 1b than in FIG. 1a, assuming that its dimension in the direction perpendicular to the plane of the drawing remains the same. The change in footprint depends on the thickness T of collimator 12 and on the angle of central ray of beam 14 relative to collimator 12. In a typical case of a medical x-ray machine using a tantalum collimator 12 at thickness T=1 mm and a focal spot-to-collimator distance of about 12 inches, the footprint can vary by about 6%.
This change in footprint area during a scan can introduce errors, because the central part of the image plane would receive more x-ray exposure than it should. For example, if the scanning beam has a substantially constant square cross-section when entering the collimator, its footprint area would be greater at the center of the scanline and for scanlines at the center of the image plane (assuming the scanning beam is normal to the image plane at its center). If the scanning beam is rectangular, with its small dimension in the direction of the scan, and has a long dimension equal to the corresponding dimension of the image plane, the beam footprint area would be greater at the center of the image plane in the scanning direction.
In addition, in a scanning structure such as illustrated in FIGS. 1a and 1b, the size of the focal spot can vary with position of the footprint along the scan line, because different parts of the x-ray cone would be used at different times. This can introduce additional errors.
One proposal for reducing such errors is to pivot the fore collimator about the x-ray focal spot. The scanning beam thus would maintain a fixed angle to the fore collimator, and the same focal spot would be used throughout the scan. However, this would still cause error, because the footprint area would still vary with position in the image plane, albeit due to another mechanism. In a particular x-ray machine, this variation can be about 1.5%. This proposal is illustrated in FIGS. 2a and 2b, where collimator 12 moves along a curved line 18, as though it is pivoted at focal spot 10, and keeps the same angle with the central ray of scanning beam 14.
An improved way to further reduce or eliminate such errors, in accordance with one embodiment of the invention, is to not only pivot the fore collimator about the focal spot of the x-ray source but also to vary its distance from the focal spot and thereby to vary its cross-sectional area in order to keep the beam footprint on the image plane from changing in area during the scan. Stated differently, this aspect of the invention pertains to controlling the scan such that the footprint area does not change substantially with position in the image plane. In one embodiment, the collimator pivots about the focal spot and maintains its angle to the scanning beam while moving generally along a straight path. In another, the movement is along a curved path, to further reduce errors or to completely eliminate them.